Semiconductor devices are very small and delicate, and must be protected from physical and environmental damage. Traditionally, the devices have been enclosed in a metal can, encapsulated by covering or potting with an organic material, or encapsulated in plastic by casting or molding. The latter method, known as transfer molding, places the semiconductor device, which has been electrically connected to a lead structure, lead frame, or circuit carrying substrate, in a mold cavity. A thermoset material is molded around the semiconductor device to form a solid monolithic unit that is sealed from environmental damage and is rugged enough to withstand physical damage while being assembled onto a circuit board. This technology may be used to form a package for an individual component, or may be used to encapsulate a semiconductor device that has been mounted directly on a circuit carrying substrate.
The molding machine is a hydraulically operated transfer mold. The molding resin is formed into a mass of precisely calculated size and shape. The amount of molding material is critical, in that there must be adequate material to completely fill the mold cavity so that no voids are left in the molded semiconductor packages. Excess molding material forces the mold to open and create flash around the edges of the molded packages, necessitating undesirable deburring and touch up operations. The mold resin is heated by a high frequency preheater and is moved into a part of the mold cavity known as the pot. The resin is further heated inside the pot and is injected into the individual mold cavities under pressure to flow around the semiconductor devices and form the molded package. A plunger forces the resin through the runners and gates, into the cavity portion of the mold. During injection and flow, the molding material begins to cure and continues curing for a predetermined time after flow has stopped. After the specified curing time, the mold is opened and the molded packages are taken out of the mold tool and separated from the runners and gates.
Molding resins used for semiconductor use are classified as thermoplastic or thermoset, with thermoset being the predominant type of resin. Thermoplastics typically exhibit problems such as melting or blistering during soldering of the semiconductor package, and the high moisture uptake of the resin leads to loss of dielectric properties. Silicone, epoxy and silicone modified epoxy resins are the thermoset resins used for encapsulating semiconductor devices. Silicone was originally used because of high heat resistance and high purity. Silicone modified epoxies offered high thermal resistance, with the advantage of lower price. However, silicones and silicone-epoxy resins have essentially been replaced by epoxies because of the lower price and higher reliability of modern epoxy resins.
Early epoxies were based on bisphenol A (also known as p,p'-dihydroxydiphenyldimethylene)-epichlorohydrin with an acid anhydride used as a hardener or curing agent. Resins derived from bisphenol A and epichlorohydrin can be described by the following general formula: ##STR1##
This idealized formula shows two epoxide end groups. In practice, side reactions intervene so that commercial resins average between 1.9 and 1.3 epoxide end groups per molecule. Commerical epoxy resins of this type are represented by Araldite 6010 (Ciba-Geigy), Epi-Rez 510 (Hoescht-Celanese), Epon 828 (Shell), and Epotuf 37-140 (Reichold).
The introduction of low pressure transfer molding techniques has resulted in the broad use of phenol or cresol novolac-modified epoxy resins. The epoxy novolac resins combine the reactivity of the epoxy group and the thermal resistance of the phenolic backbone. They are synthesized by reacting epichlorohydrin with novolac resin. The latter is obtained by condensing phenol with formaldehyde under acidic conditions and at formaldehyde-to-phenol molar ratios between 0.5 to 0.8. Epoxy-novolac resins can be represented by the following structural formula: ##STR2##
The high epoxide functionality, in comparison with bisphenol A- epichlorohydrin resins, results in a higher crosslink density and hence improved heat resistance. The novolac resins can also be prepared from substituted phenols such as cresols, polyhydroxy phenols such as resorcinol, or polyaromatic phenols. The curing mechanism of these resins is quite complex. The very reactive epoxide groups may catalytically react with each other in the presence of Lewis acids or Lewis bases, or they may react with acids, anhydrides or amines to crosslink. In addition, the novolac resin may also crosslink.
Although the basic reactions are well understood, commercial epoxy transfer molding compounds are complex mixtures of epoxy resin, curing agent, catalyst, filler, flame retardant, coupling agent, release agent, coloring agent and stress reliever. The final properties of the cured product depend strongly on the physical and chemical interactions of the many constituents. Although the epoxy-novolac resins offer superior performance over earlier materials, they still suffer from a number of disadvantages that prevent wider use in the modern semiconductor industry. Stress relievers are required to compensate for the shrinkage of the resin during the cure cycle. Ionic contaminants in the curing agents must be totally removed to prevent corrosion of the semiconductor device. This has proven to be an expensive and difficult process. Modern electronics applications require higher heat resistance and higher moisture resistance than epoxy-novolac resins can offer. The need to add flame retardants further complicates the purity of the resin, adding to corrosion problems. When used to encapsulate a semiconductor device that has been mounted directly on circuit carrying substrates employing modified epoxy resins, the conventional epoxy-novolacs suffer from poor adhesion to the substrate and a mismatch in co-efficient of thermal expansion of the substrate.
Clearly, an improved resin system for use in transfer molding semiconductor device packages is needed.